At grade intersections of streets and roads carrying vehicle traffic, such as are common in most cities in the industrialized world, impose inherent limitations on sequencing of vehicles. In the case that two such streets intersect it is common for a traffic light to sequence through two or more phases in which traffic flows alternatively along one street and then the other. Vehicles turning at such intersections impose constraints on the efficient flow of traffic. For example, if a turning vehicle must yield to oncoming traffic, which is a common rule, then vehicles behind it not turning must wait. Because of this, it is common to place restrictions on turns at intersections to promote the more efficient flow of traffic.
In some cases turns are prohibited altogether. A sign denoting a complete prohibition of left turns is shown in FIG. 1A. Another common approach is to prohibit left turns only at certain times and for certain categories of vehicles and to allow left turns at other times. A sign denoting a time and category based restriction on left turns is shown in FIG. 1B. This sign means that between the times shown on the sign (7:00 AM to 9:30 AM and 3:00 PM to 6:00 PM, Monday through Friday), left turns are prohibited. At other times, there are no restrictions and a vehicle is permitted to turn left. Buses are permitted to turn left at any time. Time based left turn restrictions tend to be based on busy commute time, i.e. “rush hour”, and are an attempt to balance the inefficiency of a left turning vehicle blocking other vehicles with the desire to allow people to travel in the most direct route. Unfortunately, time based turn restrictions are imperfect solutions. Sometimes they will permit a left turn when doing so greatly reduces the safe and efficient flow of traffic and sometimes they prohibit a left turn when such a prohibition also reduces efficient traffic flow. Thus, existing approaches to turn restrictions lead to inefficient and potentially unsafe driving conditions. An improved method for imposing restrictions on vehicles turning at intersections is needed.